Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased, and, among these secondary batteries, lithium secondary batteries having high energy density, high operating potential, long cycle life, and low self-discharging rate have been commercialized and widely used.
Recently, the research direction of a lithium secondary battery has been toward high capacity and high output. For this purpose, it is important to allow an electrode plate to be sufficiently impregnated with an electrolyte solution in a short time during the preparation of the lithium secondary battery. In a case in which impregnability of the electrolyte solution is low, since the electrolyte solution does not reach active material particles of the electrode plate, movement of lithium ions is not only not smooth, but, accordingly, a current is also reduced. Also, if an impregnation rate of the electrolyte solution is decreased, productivity of the lithium secondary battery is reduced.
Thus, an effort to improve the impregnability of the battery is not only required, but there is also a need to maintain a system capable of mass-producing high quality batteries by excluding batteries that do not reach a certain level in advance by accurately measuring the impregnability of the prepared batteries.
Currently, as a method of evaluating the impregnability of the electrolyte solution of the lithium secondary battery, there are a method of evaluating in a formation process and a method of evaluating after the preparation of the electrode plate.
The former is a method of indirectly measuring a degree of impregnation by measuring no-load voltage (OCR) and voltage drop (IR) while performing initial charge and discharge of an already assembled bare cell. Since this method is an indirect method, accuracy is poor and the entire bare cell must be discarded when it is determined that the degree of impregnation is poor, and thus, there is a limitation in that efforts and costs of assembling the bare cell may be fruitless.
The latter includes a weight measurement method in which a weight is measured after the electrode plate is immersed in the electrolyte solution and a certain period of time has elapsed. The weight measurement method has limitations in that accuracy is poor because variation with time is extremely small, and it is difficult to quantify the variation. In particular, in this method, since the entire electrode plate prepared must also be discarded when it is determined that the degree of impregnation is poor, production costs and time are consumed, and thus, productivity may be reduced.
Thus, in order to improve yield during the preparation of the lithium secondary battery, there emerges an urgent need to set a proper electrolyte solution impregnation time before the preparation of the secondary battery, or devise evaluation criteria capable of evaluating the impregnability of the electrolyte solution which may improve accuracy while fully representing the impregnability.